Yea even I was wondering on that.
But how would zoro be stronger than Mihawk in actuality when he's using weapon weaker than Mihawk post their fight.
Out of context Idea's.
- Kozuki old dude , Forge a new Supreme grade sword for Momonusuke , I mean he built Sandai , a third grade sword , then Ame-no-Habakiri , a second grade sword , then now a new Supreme grade blade. Because Nidai Is cursed and Momonusuke is too weak to carry Kitetsu Swords.
- another idea being , Shiryu get a new sword or maybe raises his sword rank from 21 to 12 , but his sword grade is unknown and zoro fan won't entertain the idea of someone else having black blade.
- or worst scene possible , S-Hawk turn his sword Black as the time continous and they grow.